How Do I Calculate Sun Hours

Needing to know the potential output of a photovoltaic (PV) system according to the available solar energy is essential when deciding the system size required for a household. But within the performance calculations, the peak sun hours are only one consideration.

Peak sun hours are directly related to another factor known as insolation, used by installation experts. It is also worth noting that according to data from the National Energy Foundation, the output of a PV system is not affected by these figures as much as might be imagined; other factors such as the position and tilt of the panels play a much greater role in ensuring efficiency.

Peak Sun Hours

This figure is not easily calculable, as it relies on Met Office data. Peak sun hours relate to the intensity of the sunlight or solar energy. A peak sun hour is an hour in the day when the intensity of the sunlight reaches an average 1,000 w/m2. In reality, this intensity is not actually received by the solar panel, as not all this irradiance reaches the surface of the earth. To the experts, sun hours are less important than the insolation factor, which is able to take other factors into account and give a more accurate figure of the amount of sunlight received at the Earth's surface.

The peak sun hours alter according to geography; the intensity of the sun is greatest at the equator of the Earth, as it is closer to the sun. The location of the UK in the Northern hemisphere leads to a lower sun hours figure.

The average sun hours by month have been calculated by the Met Office in the UK, based on averaged data from 2000 to 2010, and are shown below:

  • January - 2 Hours
  • February - 3 Hours
  • March - 4 Hours
  • April - 6 Hours
  • May - 6 Hours
  • June - 7 Hours
  • July - 7 Hours
  • August - 6 Hours
  • September - 5 Hours
  • October - 4 Hours
  • November - 3 Hours
  • December - 2 Hours

The weather conditions cannot be guaranteed and the sun hours quoted are only representative based on a ten year average.

If you wish to estimate a rough daily output for a solar system, this can be done using the figures quoted above and the following equation:

Peak sun hours per day x PV systems rated output (Watts) x 0.85 = Estimated daily output in Watts per day.

You should bear in mind that the systems rated output is the figure that the system could produce if it received 1000W/m2 or peak sun. The figure of 0.85 allows for the fact that it will not absorb peak sun at 1000W/m2, but around about 0.85 or 85% of that.

Insolation

Insolation (incident solar radiation) is a measure of the actual amount of sunlight or solar energy that strikes the surface of the earth (or the panel), and are a more accurate figure to use. The figures vary quite a lot across the UK.

An accurate solar irradiance calculator by UK location is available online here.

Insolation is used by solar panel installation experts in order to calculate the optimal panel angles to capture sunlight, position of the panels and size of system to suit a property. The figures are usually used in specialist software tools produced by the panel manufacturers, as the calculations can be quite complex. Insolation figures help ensure that the panels are installed to receive the maximum amount of solar energy possible.

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